The basic requirements to variable-ratio transformers for arc and plasma setups consist in providing a wide range of load current adjustment, high efficiency, and uncomplicated construction, which are all interconnected. Designing a variable-ratio transformer satisfying all these requirements is a difficult technical task.
The existing variable transformers meet only some of these requirements.
Known in the art is a variable-ration transformer for arc and plasma setups, in which the magnetic core comprises two legs and three yokes - the upper middle, and lower ones. The primary winding and a part of the secondary winding are arranged in the window formed by the legs, the middle and lower yokes, while in the window formed by the legs, the middle and upper yokes, the second part of the secondary winding is located. The transformer also comprises a load current regulating means which is composed of bias windings positioned on the middle and upper yokes. By adjusting the current in the bias windings, a respective yoke is saturated, and by this the second part of the secondary winding is either included in or excluded from the magnetic flux circuit.
This variable-ratio transformer is deficient in that the structure of the transformer is too complicated due to the two yokes and bias windings.
The specific consumption of materials of this transformer is too high since the second part of the secondary winding is placed too far away from the primary winding and the first part of the secondary winding. Moreover, this arrangement of the transformer components is one of the contributing factors affecting the current adjustment range which is far too narrow.
And, finally, introduction of the bias windings which consume a substantial portion of the input power results in lower efficiency of the transformer.
Also known in the art is a variable-ratio transformer for arc and plasma applications, comprising a magnetic core composed of a main part formed by two yokes and legs in accordance with the number of the transformer phases, and an additional part located on the side of one of the yokes of the main part of the magnetic core, primary and secondary windings positioned on the main and additional parts of the magnetic core, and a means for regulating the load current flowing through the secondary winding.
In this variable-ratio transformer, the primary winding and the first part of the secondary winding are located on the main part of the magnetic core, while the second part of the secondary winding is disposed on the additional part of the magnetic core. The additional part of the magnetic core is made as two L-shaped elements, one element being placed stationary in relation to the main part of the magnetic core, and the other element being composed of two sections, one stationary and the other movable in relation to the main part of the magnetic core, in order to provide an adjustable non-magnetic gap between the stationary L-shaped element and the movable section of the second L-shaped element. The second part of the secondary winding of the transformer envelops this non-magnetic gap.
The load current regulating means is a screw with a handle which can be turned to move the movable section of the second L-shaped element and thereby increase or shorten the non-magnetic gap. Correspondingly, the inductive impedance of the second part of the secondary winding can be either reduced or increased in order to regulate the load current of the transformer.
This transformer is deficient in that in order to widen the control range thereof, the number of turns in the second part of the second winding has to be increased, which is a serious limitation to the transformer effective range because of the specific material consumption and overall dimensions.
Moreover, since the second part of the secondary winding of the transformer envelops the non-magnetic gap, additional losses due to leakage fields are inevitable, and this seriously affects the efficiency of the transformer.
One more disadvantage consists in that the transformer contains two L-shaped elements, which makes its structure too complicated.